The White House, the US Department of Defence and the CIA did not immediately respond to requests for comment.

Obama had sought to influence global guidelines for the use of drone strikes as other nations began pursuing their own drone programs.

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A US Predator drone.Credit:AP

The United States was the first to use unmanned aircraft fitted with missiles to kill militant suspects in the years after the September 11, 2001, attacks on New York and Washington.

Strikes by missile-armed Predator and Reaper drones against overseas targets began under former President George W. Bush and were expanded by Obama.

Critics of the targeted killing program question whether the strikes create more militants than they kill. They cite the spread of jihadist organisations and militant attacks throughout the world as evidence that targeted killings may be exacerbating the problem.

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In July, the US government accepted responsibility for inadvertently killing up to 116 civilians in strikes in countries where America is not at war.

The US military in Kandahar, southern Afghanistan uses Predator drones against their enemy. Credit:Getty